(1) Field of the Invention
The present invention relates to moving picture coding apparatuses which code a moving picture, using inter-frame prediction, and more particularly, to a technique to determine a search range utilized for estimating an inter-frame motion vector.
(2) Description of the Related Art
In picture compression techniques utilizing correlation between pictures in a moving picture, such as the MPEG (Moving Picture image coding Experts Group) technique, a motion vector needs to be estimated for each block to be motion compensated. A typical technique to estimate a motion vector having a high accuracy is to extend a search range in order to enhance motion vector estimation accuracy. The extended search range, however, causes processing amount per current picture to be increased (since the number of blocks to be processed times a search range equals to amount to be processed), followed by an increase in amount of memory for the processing. Thus, motion vectors need to be accurately estimated without expanding the search range (See Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-169288).
A technique in Patent Reference 1 estimates motion vectors on an entire picture in the order of inputted pictures, and then calculates motion vectors between an input picture and a reference picture in accordance with the estimated motion vectors. Based on the calculated motion vectors, the technique in the Patent Reference 1 determines a search range for estimating motion vectors on a macro block-to-macro block basis.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-169288
The recent launch of digital broadcasting by broadcasting satellites and with terrestrial waves increases high demand for compression coding on a high-resolution HD (High Definition) format picture (referred to as a “HD picture” hereinafter).
The HD picture, meanwhile, has approximately six times as many pixels as a conventional SD (Standard Definition) picture. Thus, processing amount calculated by multiplying the number of blocks to be processed with a search range is required in order for the HD picture to achieve motion vector estimation accuracy as high as the motion vector estimation accuracy of the SD picture. In addition, the H. 264 standard includes as many as seven types of motion compensation sizes, so that the HD picture requires three times as much processing amount as the MPEG-2 standard requires. Thus, compression coding on the HD picture by the H. 264 standard requires several tens of times as great the amount of blocks as coding on the conventional SD picture.
Typical motion vector searching techniques narrow search ranges by estimating motion between two pictures in input pictures, and then determining motion vector search ranges on a macro block-to-macro block basis. Here, in the case where the inputted pictures are HD pictures, motion estimation of an entire picture, using all pixels, requires a significant quantity for processing since a large number of pixels are included in the HD pictures. In addition, Patent Reference 1 describes that pictures can be either reduced or eliminated in order to reduce the amount of processing. In the case where the entire picture moves slowly, (especially in the case where motion on a picture is observed approximately as great as the number of pixels with a fifty-percent reduction rate) the movement is rounded into either “0” or one pixel in a reduced picture. As a result, in the case where a p-picture three pictures before is a reference picture, a movement error between two pictures is accumulated. Thus, the search range cannot be reduced, which increases memory amount.
For example, a reduction rate of 1/16 causes a margin of error of plus or minus eight pixels. Here, when the p-picture refers to a picture three frames before, a margin of an error of plus or minus 24 pixels; namely three times, that is an error of plus-minus eight pixels time three. Since the search range should be set to include the margin of the error of plus or minus 24 pixels, an amount of processing cannot be reduced.